Measurement Considerations; Ground Loops; Figure C - Keithley 6514 Instruction Manual

C-2 General Measurement Considerations

Measurement considerations

The following measurement considerations apply to all precision measurements. Measure-
ment considerations that are unique to a particular measurement function are covered in Sections
3, 4, and 5. Table 2-5 (located at the end of Section 2) lists all measurement considerations, and
indicates where to find detailed information on them.
For comprehensive information on all measurement considerations, refer to the Low Level
Measurements handbook, which is available from Keithley.

Ground loops

Ground loops that occur in multiple-instrument test setups can create error signals that cause
erratic or erroneous measurements. The configuration shown in Figure C-1 introduces errors in
two ways. Large ground currents flowing in one of the wires will encounter small resistances,
either in the wires, or at the connecting points. This small resistance results in voltage drops that
can affect the measurement. Even if the ground loop currents are small, magnetic flux cutting
across the large loops formed by the ground leads can induce sufficient voltages to disturb sen-
sitive measurements.

Figure C-1

Power line ground loops
Signal Leads
Instrument 1 Instrument 2
Ground
Loop
Current
Instrument 3
Power Line Ground